Implement insertBefore and reimplement appendChild.
Address review comments and replace add_to_doc with node_removed and node_inserted functions.
auto merge of #1136 : Ms2ger/servo/insertBefore, r=jdm
HTMLDirectoryElement should be 'dir'. Fix #1159
auto merge of #1160 : saneyuki/servo/dir, r=jdm

@Ms2ger?
Implement basic overflow computation. Closes #1148.

This adds just 4 ms out of ~120 ms on the rainbow page.
auto merge of #1149 : pcwalton/servo/store-overflow, r=kmcallister

This adds just 4 ms out of ~120 ms on the rainbow page.

r? @kmcallister